The present invention relates to a common method of detecting abnormalities of various key components of an electrostatic image-recording machine (e.g., a copying machine, a printer, etc.) and to an image-recording machine equipped with such abnormality-detecting function.
First, structure and operation of a typical conventional electrostatic copying machine are explained referring to FIG. 13 for easier understanding of the background of the present invention. The copying machine includes a reading part and a recording part within a body frame 129.
The recording part of the copying machine includes a main charger 102, a developing section 106, image-transferring section 107, separating section 108, drum-cleaning section 110 and discharging section 111, all of which are arranged around a photoreceptor drum 101 in the order of its rotation. Between the main charger 102 and the developing section 106 is arranged a space for an exposure section 103.
The drum 101 is a cylindrical body (usually made of aluminum or aluminum-alloy) coated on its surface with a photosensitive compound (e.g. vapor-deposited selenic compound). The main charger 102 is composed substantially of a charging wire 102b (usually made of tungsten) strained parallel to the drum axis and very close to the drum surface, and a case 102a covering the charging wire 102b at the back along the length. The charging wire 102b is applied a high voltage of 5-6kV to give the drum surface (photosensitive layer) a static potential of 600-800 V. The developing section 106 includes a toner hopper 112, regulator roller 113, mixing roller 114 and a developing roller 115. The image-transferring section 107 and the separating section 108 have almost the same structure as the main charger 102. The drum-cleaning section 110 includes a rubber blade 117 to wipe off toner powder on the drum surface, and the discharging section 111 includes a discharge lamp 118 whose irradiation discharges the photosensitive layer of the drum surface.
The reading part of the copying machine includes a moving optical system 128 which includes two independently movable units: an illumination unit 119 and a mirror unit 125. The illumination unit 119 is composed of an exposure lamp 120, elliptic reflector 121, auxiliary reflector 122 and a first reflector mirror 123. A slit 133 is formed between the elliptic reflector 121 and the auxiliary reflector 122. The mirror unit 125 is composed of a second reflector mirror 126 and a third reflector mirror 127.
Copying process by this copying machine is briefly explained. When an original 124 is placed on a glass plate 130 at the top of the body frame 129, pressed by a cover plate 131 with a buffer sponge 132 and the operator presses the copy-start switch, the illumination unit 119 runs from left to right along the glass plate 130 at a constant speed V, and the mirror unit 125 follows the illumination unit at half the speed V/2. The light of the lamp 120 is reflected by the original 124 and passes through the slit 133. The slitted light is reflected by the first, second and third mirrors 123, 126 and 127, passes through a fixed lens 134 and is further reflected by a fourth reflector mirror 135 to be brought to the exposure section 103 of the drum 101.
Since the mirror unit 125 runs at half speed of the illumination unit 119, the length of the light path from the surface of the original 124 to the surface of the drum 101 is kept constant throughout the reading process.
When the operator presses the copy-start switch, the recording part is also activated. The drum 101 starts rotating at a constant speed clockwise in FIG. 13, and the surface photosensitive layer of the drum 101 is charged at around 700 V by the main charger 102. The image of the original 124 is recorded at the exposure section 103 for producing a latent image on the charged photosensitive layer. Toner powder in a cassette 140 attached to the hopper 112 comes down through the regulator roller 113, and is mixed with carrier powder (usually iron powder) by the mixer roller 114. The developing roller 115 is charged at a predetermined bias voltage to attract the mixed powder around it, and only the toner is transferred from the developing roller 115 to the drum 101 by the voltage difference between them, by which the latent image is developed to a real image. The real image of toner is transferred onto a sheet of paper 142 which is supplied one by one by a pair of resist rollers 116 at the transferring section 107. The charging wire 108b of the separating section 108 applies alternating (AC) electrical field onto the drum 101 to separate the sheet 142 stuck to the drum 101 by electro-static force. Thus the copy of the original 124 is obtained.
After the sheet 142 is separated, the toner remaining on the drum surface is wiped off by the rubber blade 117 of the cleaning section 110, and the photosensitive layer of the drum 101 is discharged at the discharging section 111 by the discharge lamp 118.
The whole electrical system of the copying machine is controlled by a central processing unit (CPU) (not shown in FIG. 13) which uses a micro-computer.
The image-recording machine has some key components, and it is important for normal use of the machine to detect abnormalities of such key components.
One of the key components is the charging wire 102b, 107b or 108b. Since it has a very high voltage potential when activated, it tends to collect environmental dusts. Therefore, cleaning of the charging wire is necessary from time to time. Conventionally, such wire cleaning is performed when the operator notices it from the quality of the copy image, which is awkward and sometimes too late to maintain good copying quality.
The charging wire is applied such high voltage by a high voltage unit (HVU). The failure of the charging wire (e.g., brake or short circuit of the wire) or of the HVU is detected by a specially provided circuit in the HVU and is informed to the CPU of the image-recording machine by an appropriate alarm signal line. This requires the additional detecting circuit in the HVU and additional signal line between the HVU and the CPU, both increasing the cost of the machine and decreasing the reliability of the electrical system.
The charging wire 108b of the separating section 108 is different among the three wires because it is applied an alternating voltage (AC voltage) to separate the sheet 142 statically attached to the drum 101. Precisely saying, the AC voltage (normally +/-5 kV) of the separating wire 108b is biased by a small amount ("shift-bias", about 0.1 kV) to compensate for the tendency of the photosensitive layer of the drum 101 to shift to negative charging.
Another key component is the discharge lamp 111. Insufficient discharging of the drum 101 caused by deposition of toner powder on the lamp surface, or failure of discharging due to malfunction of the lamp 111, will lead to an accumulated pile-up of charge on it, which results in foggy background of the recorded image.
These abnormalities of the discharge lamp 111 is conventionally detected by the devices as shown in FIGS. 14A and 14B. In the device of FIG. 14A, the current to the discharge lamp 111 is allowed or stopped by the CPU 150 using a switching triac 154, and when the current flows, it is monitored by the CPU 150 using a photocoupler 152. When the amount of current falls below a certain level, the discharge lamp 111 is judged abnormal. In FIG. 14B, the light from the lamp 111 is directly sensed by a photosensor 156. In any case, the detecting device require an additional component (photocoupler 152 or a photosensor 156) which increases the cost of the machine.
Still another key component is the developing roller 115. The bias voltage for the developing roller 115 must be strictly controlled to obtain a clear toner image on the drum 101 because, as explained before, the voltage difference between the roller 115 and the drum 101 determines the amount of toner transferred.
Conventional abnormality detecting method for this bias voltage for the developing roller 115 is the same as that for the charging wires; i.e., to provide an abnormality detecting circuit in a controller for giving the bias, and the abnormality signal is sent to the CPU using an appropriate alarm-signal line provided besides the control-signal line between the CPU and the bias controller.